The present invention relates to an optical recording medium having a plurality of information recording surfaces each permitting information reproducing by virtue of a laser beam.
There have been in actual use several kinds of optical recording media each of which has a plurality of information recording surfaces. For example, a DVD (digital versatile disc) has two to four information recording surfaces. A user can use a reproducing device to reproduce information from any of these information recording surfaces without having to turn the disc over.
FIG. 12 is an enlarged cross sectional view schematically indicating an example of a conventional optical recording medium having two information recording surfaces (hereinafter referred to as two-disc laminated recording medium), which is adapted to allow a user to use a reproducing device to reproduce information from any of the two information recording surfaces without having to turn the disc over.
As shown in FIG. 12, the conventional two-disc laminated recording medium comprises a first substrate layer 1 formed of a light-transmissible material having a first recording surface 2 formed thereon, a first reflecting layer 3 consisting of gold (Au) or silicon carbide (SiC) formed on the first recording surface 2, a second substrate layer 7 having a second recording surface 6 formed thereon, a second reflecting layer 5 consisting of a metal such as aluminium (Al) formed on the second substrate layer 7. The first substrate layer 1 and the second substrate layer 7 are bonded together with the use of a light-setting resin such as an acrylic ultraviolet-setting resin, in a manner such that the two recording surfaces are faced each other. In fact, such light-setting resin is formed into a spacer layer 4 upon its hardening. Each recording surface includes a plurality of information pits possible to be read by a laser beam having a wave length of 650 nm, and such pits are formed on one surface of each substrate layer.
When reproducing information recorded on the above two-disc laminated recording medium, it is at first necessary to decide which information recording surface is to be reproduced. If it is decided to reproduce, information recorded on the first recording surface 2, a red laser beam having a wave length of 650 nm is converged on to the first recording surface 2, a reflected light from the first reflecting layer 3 is received so that its variation in intensity may be detected, thus reproducing the information recorded on the first recording surface 2.
On the other hand, if it is decided to reproduce information recorded on the second recording surface 6, the same laser beam having a wave length of 650 nm is directed through the first substrate layer 1, the first reflecting layer 3, the spacer layer 4, so as to be converged on to the second recording surface 6. Then, a reflected light from the second reflecting layer 6 is received through the spacer layer 4, the first reflecting layer 3 and the first substrate layer 1, so that its intensity variation may be detected, thereby reproducing the information recorded on the second recording surface 6.
In this manner, it is possible to selectively and continuously reproduce information recorded on the first and second recording surfaces 2 and 6, with the use of a laser beam being directed through the first substrate layer 1. Since it is not necessary to direct a laser beam through the second substrate layer 7, it is not required that the second substrate layer 7 be formed of a light-transmissible material.
However, in order to reproduce the information recorded on the second recording surface 6, it is required that the first reflecting layer 3 be made of a material capable of allowing a laser beam to transmit therethrough to some extent. In practice, the first reflecting layer 3 is a gold film (Au) having a thickness of about 15 nm.
Recently, people have begun to develop a new generation DVD capable of performing an information recording with a further high density using a blue laser beam having a wave length of 400-450 nm, so as to form information pits having smaller sizes which are arranged along a plurality of tracks formed with a smaller pitch. Such kind of DVD should of course be made into a type of a two-disc laminated recording medium in order to store as much information as possible.
However, with a conventional DVD as discussed above, the gold film or the silicon carbide film forming the first reflecting layer 3 has only a low reflectance with respect to a blue laser beam and has only a small light-transmissibility. Accordingly, if the first reflecting layer 3 is formed of gold (Au), it will be difficult to obtain a sufficient amount of reflected light from said first reflecting layer 3, hence making it difficult to reproduce information recorded on the first recording surface 2.
Further, since a light amount passing through the first reflecting layer 2 will become small due to its above-discussed defect, an amount of a reflected light from the second reflecting layer 5 will also be small, thus rendering it difficult to reproduce information recorded on the second recording surface 6.
In order to ensure a sufficient amount of a reflected light for information reproducing, it has been suggested that the first reflecting layer 2 be formed of a material having an adequate reflectance and an adequate light-transmissibility.
To meet the above requirements, Japanese Unexamined Patent Publication No. 9-293270 has disclosed an improved two-disc laminated recording medium whose first reflecting layer is formed into a multi-layer structure including a metal film and a dielectric film.
However, since a reflecting layer has a multi-layer structure, each sub-layer of it has to be controlled strictly, making a whole manufacturing process more difficult than usual. Further, since a metal film forming a reflecting layer is likely to be oxidized and hence corroded, some desired optical properties of a two-disc laminated recording medium will be unavoidably deteriorated.
It is an object of the present invent ion to provide an improved multi-disc laminated recording medium having at least two or three information recording surfaces, which is easy to manufacture and has an excel lent durability for a long-term use, so as to solve the above-mentioned problems peculiar to the above-mentioned prior arts.
According to the present invention, there is-provided an improved optical recording medium which comprises a first information recording surface; a first reflecting layer formed on the first information recording surface; a second information recording surface; and a second reflecting layer formed on the second information recording surface. In particular, the first reflecting layer contains a titan oxide as a main component.
According to the present invention, there is provided a further improved optical recording medium, comprising: a first information recording surface; a first reflecting layer formed on the first information recording surface; a second information recording surface; a second reflecting layer formed on the second information recording surface; a third information recording surface; and a third reflecting layer formed on the third information recording surface. In particular, each of the first and second reflecting layers contains a titan oxide as a main component.
In one aspect of the present invention, the titan oxide contained in each of the first and second reflecting layers has an 0/Ti ratio of 1.8-2.2.
In another aspect of the present invention, each of the first and second reflecting layers is in an amorphous state or consists of an aggregate of fine crystals.
In a further aspect of the present invention, each of the first and second reflecting layers contains a noble gas element with a concentration of 3 atom % or less.
In a still further aspect of the present invention, each of the first and second reflecting layers has a thickness of 150 nm or less.
In one more aspect of the present invention, the third reflecting layer is made of a metal containing an aluminium as a main component.
The above objects and features of the present invention will become better understood from the following description with reference to the accompanying drawings.